Introduction to Web Applications

Before discussing the ASP.NET Core and its features, let us understand the fundamentals of web application development.

Remember this principle: If you want to be an expert at something, you need to be very good at the fundamentals.

How Web Applications Work

All web applications, irrespective of whether they are built using ASP.NET MVC (MVC stands for Model-View-Controller), which is actually inspired by the success of Ruby on Rails, or any other new shiny technology, work on the HTTP protocol. Some applications use HTTPS (a secure version of HTTP), where data is encrypted before passing through the wire. But HTTPS still uses HTTP.

Symmetric encryption is the conventional method to ensure the integrity of the data transferred. It makes use of only one secret key, called a symmetric key, for both encryption and decryption. Both the sender and receiver possess this key. The sender uses it for encryption, while the receiver uses it for decryption. Caesar's Cipher is a good example of symmetric encryption.

Asymmetric encryption makes use of two cryptographic keys. These keys are known as public and private keys. The information to be sent is encrypted by the public key. The private key is used to decrypt the information received. The same algorithm is behind both of these processes. The RSA algorithm is a popular algorithm used in asymmetric encryption.

Encryption ensures the integrity of the data transferred by making use of cryptographic keys. These keys are known only by the sender and the receiver of the data being transferred. This means that the data won't be tampered by anyone else. This prevents man-in-the-middle attacks.

 

What is the HTTP Protocol?

A protocol is nothing but a set of rules that govern communication. The HTTP protocol is a stateless protocol that follows the request-response pattern.

HTTP stands for HyperText Transfer Protocol and is an application protocol which is designed for distributed hypermedia systems. HyperText in HyperText Transfer Protocol refers to the structured text that uses hyperlinks for traversing between the documents. Standards for HTTP were developed by the Internet Engineering Task Force (IETF) and the World Wide Web Consortium (W3C). The current version of HTTP is HTTP/2 and was standardized in 2015. It is supported by the majority of web browsers, such as Microsoft Edge, Google Chrome, and Mozilla Firefox.

HTTP/2's Edge over HTTP/1.x

At a high level, HTTP/2:

• Is binary, instead of textual
• Is fully multiplexed, instead of ordered and blocking
• Uses one connection for parallelism
• Uses header compression to reduce overhead
• Allows servers to push responses proactively into client caches

Request-Response Pattern

Before talking about the request-response pattern, let's discuss a couple of terms: client and server. A server is a computing resource that receives the requests from the clients and serves them. A server, typically, is a high-powered machine with huge memory to process many requests. A client is a computing resource that sends a request and receives the response. A client could typically be any application that sends the requests.

Coming back to the request-response pattern, when you request a resource from a server, the server responds to you with the requested resource. A resource could be anything—a web page, text file, image, or another data format.

You fire a request. The server responds with the resource. This is called a request-response pattern.

Stateless Nature of HTTP

When you request for the same resource again, the server responds to you with the requested resource again without having any knowledge of the fact that the same was requested and served earlier. The HTTP protocol inherently does not have any knowledge of the state of any of the previous requests received and served. There are several mechanisms available that maintain the state, but the HTTP protocol does not maintain the state by itself. We will explain the mechanisms to maintain the state later.

Advantages to HTTP

Here are the few advantages of using HTTP protocol:

• HTTP is a text-based protocol that runs on top of TCP/IP
• HTTP is firewall-friendly
• HTTP is easier to debug since it is text based
• All browsers know about HTTP. Thus, it is extremely portable on any device or any platform
• It standardizes the application-level protocol into a proper request–response cycle

With TCP/IP, everybody has to invent their own application protocol.
HTTP is traditionally not full duplex, but with HTML5 we can use Web Sockets to upgrade HTTP connections to a full duplex connection.

Work with the Statelessness and the Request-Response Pattern

With the help of a simple practical example, let's work with the statelessness and the request-response pattern. Here are the steps:

1. Type this URL: https://en.wikipedia.org/wiki/ASP.NET_Core. This is a Wikipedia web page about ASP.NET Core.

We'll talk about ASP.NET later in this chapter.

2. From the preceding URL, the browser fires a request to the Wikipedia server.
3. The web server at Wikipedia serves you the ASP.NET Core web page.
4. Your browser receives that web page and presents it.
5. Now, request the same page again by typing the same URL again (https://en.wikipedia.org/wiki/ASP.NET_Core) and pressing Enter.
6. The browser again fires the request to the Wikipedia server.
7. Wikipedia serves you the same ASP.NET Core web page without being aware of the fact that the same resource was requested previously.

8. Here's a screenshot from the Wikipedia page showing requests and responses:

As mentioned earlier, there are several mechanisms to maintain the state. Let us assume, for the time being, that no such mechanism is implemented here.

Client Side and Server Side

It is necessary to understand the client side and server side of web applications and what can be done on either side. With respect to web applications, your client is the browser and your server could be the web server/application server.

The client side is whatever that happens in your browser. It is the place where your JavaScript code runs and your HTML elements reside.

The server side is whatever happens at the server at the other end of your computer. The request that you fire from your browser has to travel through the wire (probably across the network) to execute some server-side code and return the appropriate response. Your browser is oblivious to the server-side technology or the language your server-side code is written in. The server side is also the place where your C# code resides.

Let us discuss some of the facts to make things clearer:

• Fact 1: All browsers can only understand HTML, CSS (Cascading Style Sheets), and JavaScript, irrespective of the browser vendor:
  • You might be using Microsoft Edge, Firefox, Chrome, or any other browser. Still, the fact is that your browser can understand only HTML, CSS, and JavaScript. It cannot understand C#, Java, or Ruby. This is the reason why you can access the web applications built using any technology by the same browser:

• Fact 2: The purpose of any web development framework is to convert your server-side code to HTML, CSS, and JavaScript:
  • This is related to the previous point. As browsers can only understand HTML, CSS, and JavaScript, all the web development technologies should convert your server-side code to HTML, CSS, and JavaScript so that your browser can understand. This is the primary purpose of any web development framework. This is true whether you build your web applications using ASP.NET MVC, ASP.NET Web Forms, Ruby on Rails, or J2EE. Each web development framework may have a unique concept/implementation regarding how to generate the HTML, CSS, and JavaScript, and may handle features such as security performance differently. But still, each framework has to produce the HTML, because that's what your browsers understand.

Programming Styles – RPC versus REST

Basically, there are two common styles when programming HTTP: Remote Procedure Calls and REST. Let's look at each here:

• Remote Procedure Calls: In the RPC style, we usually treat HTTP as a transport medium and do not focus on HTTP itself. We are simply piggybacking on HTTP. Our service provides some set of operations that are callable directly. In other words, from our client, we call methods as if we are calling normal methods and passing parameters. Usually, RPC is applied via SOAP (Simple Object Access Protocol), which is another XML protocol that runs on top of HTTP. RPC was popular before 2008, and these days the RESTful approach is more popular, since RPC style introduces more coupling between client and server.
• REST: REST stands for Representational State Transfer. In REST, we use URLs to represent our resources, such as https://api.example.com/books/. This URL is basically an identifier for a book collection. And for example, the following could be an identifier for the book with ID 1: https://api.example.com/books/1.

Then, we use HTTP verbs to interact with these resources. HTTP verbs and HTTP methods are synonyms. The available methods in HTTP are GET, HEAD, POST, PUT, DELETE, TRACE, OPTIONS, CONNECT, and PATCH. So, when we make an HTTP request with GET, we are basically asking the web server to return that resource representation. And that representation can change, even for each request.

The server can return XML for one request and JSON for another, depending on what a client accepts, which is specified by the Accept header.

Why do we need REST? It is all about standardization. Suppose that we access a resource by using the GET verb; we inherently know that we are not altering anything in the server. Similarly, when we send a request via PUT, we inherently know that the requests are idempotent, meaning duplicate requests won't change anything to the same resource. Once we have this standard established, our application behaves like a browser. Just like a browser does not need documentation of an API while walking through the pages, our applications will not need documentation, but only adhere to the standards.

Working with HTTP Methods

HTTP defines methods (sometimes referred to as verbs) to indicate the desired actions to be performed on the identified resources. It is a part of HTTP specification. Even though all the requests of the HTTP protocol follow the request-response pattern, the way the requests are sent can vary from one to the next. The HTTP method defines how the request is being sent to the server.

The available methods in HTTP are GET, HEAD, POST, PUT, DELETE, TRACE, OPTIONS, CONNECT, and PATCH. In most of the web applications, the GET and POST methods are widely used. In this section, we will discuss these methods. Later, we will discuss other HTTP methods on a need-to-know basis.

The GET Method

GET is a method of the HTTP protocol which is used to get a resource from the server. Requests which use the GET method should only retrieve the data and should not have any side effect. This means that if you fire the same GET request again and again, you should get the same data, and there should not be any change in the state of the server as a result of this GET request.

In the GET method, the parameters are sent as part of the request URL and will therefore be visible to the end user. The advantage of this approach is that the user can bookmark the URL and visit the page again whenever they want. An example is https://yourwebsite.com/?tech=mvc6&db=sql.

We are passing a couple of parameters in the preceding GET request. tech is the first parameter, with the value mvc6, and db is the second parameter, with the value sql. Assume your website takes the preceding parameters with values and searches in your database to retrieve the blog posts that talk about mvc6 and sql before presenting those blog posts to the user:

The disadvantage of the GET method is that, as the data is passed in clear text in the URL as parameters, it cannot be used to send sensitive information. Moreover, most browsers have limitations on the number of characters in the URL, so, when using GET requests, we cannot send large amounts of data.

The POST Method

The POST request is generally used to update or create resources at the server, as well as when you want to send some data to be processed by the server. Especially in the context of REST, it is more accurate to consider POST as a process rather than Create.

Data is passed in the body of the request. This has the following implications:

• You can send relatively sensitive information to the server, as the data is embedded in the body of the request and it will not be visible to the end user in the URL. However, note that your data is never truly secure unless you use HTTPS. Even if you send the data within the request body, without HTTPS, it is very easy for someone in the middle to eavesdrop on your data.
• As the data is not sent through the request URL, it does not take up space in the URL, and it therefore has no issues with the URL length limitations:

As we have covered the fundamentals, we can now proceed to discuss ASP.NET.

List of Important Methods

Before we discuss the HTTP methods, let's review three aspects of HTTP verbs:

• Idempotency: Idempotency is an important concept in HTTP calls. In idempotent requests, you can change the server-side state (however, only once). That is, if you make multiple idempotent requests to the server, the net effect will be as if you have done one request.

• Safety: Safe requests simply do not cause any side effects. They are only used to retrieve data. By side effects, we refer to any persistent changes in memory or database or any other external system. Registering a user is a side effect. Making a money transfer is a side effect. But viewing user information is not a side effect.
• Cacheablity: Server or client or proxies can cache the responses for the requests.

The following table lists the important HTTP methods and their aspects:

Method	Description	Idempotent	Safe	Cacheable
GET	Reads a resource.	Yes	Yes	Yes
POST	Creates a resource or triggers a process.	No	No	No
PUT	Puts something onto a resource ID. Overrides if something exits. Not to be confused with an update.	Yes	No	No
PATCH	Updates a part of a resource.	No	No	No
DELETE	Removes a resource.	Yes	No	No

 

In the preceding table, we can see that the GET method is the only safe method. And that's why, for example, search engines like Google only use GET methods to scan our side. Adhering to this standard makes sure nothing is changed during a search engine scan.

Other Methods

Some of the other notable methods are as follows:

• CONNECT: This is used for HTTP tunneling for security reasons. It's not common in typical web applications and services.
• TRACE: It is used for debugging purposes. It's not common in typical web applications and services.
• OPTIONS: By using the OPTIONS verb, we can query which methods are supported by the web server for that resource.

Here's some part of the response after the OPTIONS method is invoked:

HTTP/1.1 200 OK
Allow: OPTIONS, GET, HEAD, POST

Activity: Working with the Request-Response Pattern

Scenario

Your company wants you to monitor the network traffic of their website. Here, we use https://www.google.com/ as a reference.

Aim

To check the request-response pattern for https://www.google.com/.

Steps for completion

1. Open your favorite browser.
2. Hit F12 to open developer tools.
3. Then, click on the Network tab.
4. Next, go to https://www.google.com/.
5. Study the header body for request and response.

You should see something similar to what is shown in the following screenshot: